Fuels for vehicles, in particular diesel fuels, exhibit to some extent significant differences in quality depending on the region (for example in the U.S., a significant variance in the cetane number and in the content of polyaromatics is possible). In addition to diesel fuel based on petroleum, they may also contain components of alternative fuels. For example, EU Directive 2003/30/EG prescribes a market share of 5.75% of alternative fuels. For example, bio-diesel based on rapeseed (RME—rape methyl ester), synthetic diesel fuels from biomass (BTL—biomass to liquid) or synthetic diesel fuels made from natural gas (GTL—gas to liquid) are added to diesel fuel based on petroleum. For example, GTL has already been established in the market as a 5% additive to mineral diesel fuel. RME as well as synthetic diesel fuel have different fuel densities and different specific calorific values than mineral diesel fuel. Due to the density and the differences in the specific calorific values, an excess quantity may be necessary for achieving a comparable engine torque and the same rotational speed. For unadapted data calibration, changes in the injection quantity may result in a higher exhaust gas temperature and/or for example in the impinging of the injected jet on the cylinder wall/combustion cavity of the piston, thus causing damage to the components and/or a deterioration in the exhaust gas characteristics. In addition, the proportion of combustion soot as well as the proportion of nitrogen oxide emissions is changed by the fuel additives, and often even improved compared with mineral fuel. These improvements cannot be exploited to the full extent. When using such fuels, the engine calibration is generally not tuned optimally with respect to emissions, fuel consumption, noise, or regarding the handling characteristics of a vehicle equipped with a similar internal combustion engine.
German patent application DE 10 2007 01 928.0 (which is not a prior publication) of the applicant discusses a method and a measuring device for determining the properties of liquids used in vehicles, in particular for determining the quality of fuel for vehicles, where the liquid, i.e., the fuel, is put into a rotational motion within a rotationally symmetric container, and where the characteristics of the liquid, in particular the quality of the fuel, are inferred from the change in the free surface of the liquid, in particular a lowering or rising of the surface of the liquid at the container wall and/or in the area of the axis of symmetry of the container.
German patent application DE 10 2007 01 928.1 (which is also not a prior publication) of the applicant discusses a method for monitoring the properties of the fuel of an internal combustion engine, in particular a self-igniting internal combustion engine, in which the temperature of the fuel is determined at least at one location of a fuel system and the instantaneous rate of decomposition of the fuel is inferred from this temperature.
In the near future, it is to be expected for several reasons that the fuel stability during the operation of the vehicle will be affected more than is the case today. On the one hand, the injection pressures will rise further. This results in an increase in the maximum fuel temperature and thus in a more rapidly declining stability of the fuel. On the other hand, larger proportions of bio-diesel will be added to the diesel fuel in the future. This proportion may be as high as 30%. Increased quantities of bio-diesel fuel reduce the resistance of the fuel against elevated temperatures.